On guard ... police watch passengers queue at the security checkpoint at the Rhein-Main airport in Frankfurt, Germany, after. US intelligence officials expressed concern that al Qaeda is trying to develop a bomb that could go undetected through airport security.

HAS jihadist group al Qaeda evolved and developed a new “stealth suicide bomber” capable of slipping past all airport security measures?

First they tried innocuous-looking water bottles filled with clear-liquid explosives.

Then they tried stuffing detonators in shoes.

Even underwear was turned into an explosive threat.

Most of the Islamic fanatics’ deviant weapons have so far been caught “at the gate” by airport security systems. But a few disturbing examples have actually made it aboard a plane.

Now fears are centred about nonmetallic explosives being surgically implanted inside a suicide bomber’s body — where it would be invisible to metal detectors, explosive taste-tests and even physical pat-downs.

Obsolete? Fears have been raised that surgically implanted explosives will evade common airport security devices and physical pat-downs.Source:AP

Penetrative security ... controversial new full-body imaging scanners have been introduced in airports in recent years to reveal hidden weapons.Source:Supplied

THREAT WARNING

Despite the rising speculation among terror-focused media analysts, US intelligence agencies reportedly say there is no indication a new “stealth bomb” has been created. But Wednesday’s urgent call for tighter security measures at foreign airports with direct flights to the US has renewed speculation as to what trick al Qaeda and its affiliates will come up with next.

A US counterintelligence official told media that there had been indications that bomb makers among al Qaeda operatives based in Yemen had travelled to Syria to link up with another jihadist organisation there — the Nusra Front.

The movements of key al Qaeda personnel have sparked concern — although no confirmations — of a new attack plan. Enhanced security measures were initiated as early as a month ago based on these movements, the US official said.

And al-Qaeda is believed to be under mounting pressure to prove it is still relevant as eyes turn towards the success of rival jihadist organisation ISIS.

TERROR INNOVATION

The concept of surgically-implanted explosives has its challenges, but its potential to bypass security screens continues to heighten fears.

At the centre of such a plot would be pouches of a clear liquid explosive, invisible to metal detectors and X-ray scanners.

“A surgeon would open the abdominal cavity and literally implant the explosive device in among the internal organs,” Dr Mark Melrose explained to US ABC News.

“The blast is blunted by the body itself. Ninety per cent of the human body consists of water, which is a very effective blast absorber; and ... (shape) the blast to go up to the ceiling rather than across the room,” historian Michael Burleigh told the Daily Mail.

2006: A plot involving a “liquid bomber” was foiled before it reached a plane. Explosive hydrogen peroxide was the threat, hidden inside drink bottles. A simple electrical charge from a hot-wired everyday personal device was the intended detonator.

2001: In December, British-born Richard Reid attempted to blow up a flight from Paris to Miami. Only sweaty feet saved the lives of the Boing 747’s passengers from the “shoe bomber”, as this caused the plastic explosives embedded in the soles of his shoes to become damp.

US officials are reported as saying that al Qaeda and Nusra are believed to have been testing new bomb designs in Syria. Nusra is one of the largest Islamic groups fighting to overthrow Syria’s President Bashar al-Assad.

WESTERN SUICIDE BOMBERS

The public warning is just the latest escalation of the threat assessment.

It is understood that the present of the Yemeni bombmakers in Syria has raised fears US citizens and other westerners may be among potential new suicide bombers.

Double-edged sword ... western passport holders have flocked to join the fight against Syria’s President Assad. Fears are that, after serving with extremist jihadist groups, these warriors will now turn against their homelands.Source:AFP

At the weekend, US President Barack Obama warned that “battle-hardened’’ Europeans who embrace jihad in Syria and Iraq threaten the United States because their passports mean they can enter without a visa.

In the past few years many westerners have travelled to the war-torn country to help US-backed jihadist groups fight President Assad’s forces. The fear is these US or British passport holders will now turn their jihad against their home countries.

UNDERLYING THREAT

Some terror analysts have dismissed the “invisible suicide-bomber” scare.

AFP reports sources as saying the security move was likely linked to al Qaeda passing on bomb-making expertise to militants fighting in Syria.

The fear is that militants with Western passports could then bring such skills back home with them before launching an attack, they said.

The airports involved in the heightened security scare are located in the Middle East and Europe and were targeted “based on real-time intelligence,’’ an official at the Department of Homeland Security said Wednesday.

On guard ... police watch passengers queue at the security checkpoint at the Rhein-Main airport in Frankfurt, Germany, after. US intelligence officials expressed concern that al Qaeda is trying to develop a bomb that could go undetected through airport security.Source:AP

But since then US counterterrorism officials have been attempting to hose-down fears, insisting that the new alert had nothing to do with the success of jihadist organisation ISIS in Iraq. They also said there was no specific threat targeted at the upcoming July 4 holiday.

Instead, the extra security represents an “abundance of caution,” a US official said.

“People should not overreact to it or over-speculate about what’s going on, but there clearly are concerns centred around aviation security that we need to be vigilant about,” Homeland Security Secretary Jeh Johnson said late Wednesday on MSNBC.

DANGER AIRPORT NAMED

But reports of just such a specific threat has since emerged: The US embassy in Uganda warned of a “specific threat’’ to attack Kampala’s Entebbe international airport this morning Australian time.

The airports concerned are located in the Middle East and Europe and were targeted “based on real-time intelligence,’’ according to an official at the Department of Homeland Security who spoke on condition of anonymity.